The New York Police Department (NYPD) is developing a new technology that would allow police to see whether a person was carrying a weapon from a distance, NYPD Commissioner Raymond Kelly said during his State of the NYPD address on Tuesday.

The technology will use a vehicle mounted device to detect radiation emitted from a person’s body. These radiation waves, called terahertz (THz) radiation, can travel through fabric and packaging, but cannot travel though metal. Images captured by the device could reveal a gun underneath a person’s clothing.

Kelly says the new technology shows promise as “a way of detecting weapons without a physical search.” During the first nine months of 2011, 88 percent of people subjected to stop-and-frisks were completely innocent, according to NYPD data compiled by the New York Civil Liberties Union (NYCLU).

The NYPD and the Department of Defense have been working on the device for about three years. Currently, it can scan a person from three to four feet away. Kelly hopes to increase that capability to 25 meters–an ambitious goal, according to Arizona State University physics professor Peter Rez.

Rez says the technology described by Kelly is passive THz scanning and wouldn’t be able to detect a gun even at 30 feet.

“It’s just detecting the tail end of heat radiated from objects. Even with active THz or mm (millimeter wave) scanning it would be hard. The mm wave body scanner is making an image of a person 1 foot away, not 30 feet,” he said by email.

Scientists have been working to improve reading THz waves for medical imaging and security screening applications. At Berkeley National Laboratory, researchers are working with a revolutionary material called grapheme that better absorbs THz waves. Researchers at Harvard are developing a device similar to what Kelly described and say THz scanning could become commercially available in the next five years.

Civil liberties advocates aren’t waiting until then to raise questions about the technology. Civil liberties lawyer Normal Siegel told the New York Daily News that he hopes the scanning devices will be accurate enough to distinguish between a gun and other metal objects; otherwise it could lead to police making unwarranted stops.

In a statement on Tuesday, NYCLU Executive Director Donna Lieberman said she finds the idea “both intriguing and worrisome.”

“On the one hand, if technology like this worked as it was billed, New York City should see its stop-and-frisk rate drop by a half-million people a year. On the other hand, the ability to walk down the street free from a virtual police pat-down is a matter of privacy,” she said.